The present invention relates generally to variable ratio roller chain drives such as are commonly employed on bicycles and, more particularly, to variable ratio roller chain drives which can be shifted under load.
Perhaps the most common variable speed ratio roller chain drive employed on bicycles is usually termed the derailleur system. The derailleur system has a stack of sprocket rings on the rear wheel, usually referred to as either free-wheel cogs or sprockets, and a stack of sprocket rings on the pedal shaft, usually referred to as chain rings. The free-wheel sprockets typically can number as many as eight, and chain rings usually number either two or three.
To compensate for the change in length of chain path as various pairs of driving and driven sprocket rings are entrained by the chain, a chain tightener is mounted in the slack pass of the chain near the rear wheel. The chain tightener typically comprises a pivoted, spring loaded pair of small sprockets S-wrapped by the chain, and is mounted on a parallelogram linkage such that it can be moved laterally, i.e., in a direction parallel to the rear wheel shaft.
The chain tightener also serves the purpose of effecting shifting, and is referred to as the rear derailleur. Thus the bicycle rider, by means of a suitable lever and cable device, laterally moves the pair of tightener sprockets comprising the rear derailleur while the chain is traveling, thereby exerting a lateral force on the chain. This causes the chain to jump in the direction of the lateral force from the sprocket about which it is wrapped to an adjacent sprocket. Since the sprocket thereby newly entrained is of a different pitch diameter, i.e., a different number of teeth, than the sprocket previously entrained, the driving ratio of the driving chain ring with reference to the driven free-wheel sprocket is changed.
Similarly, there is a lever and cable operated device called the front derailleur, which straddles the chain on its tight, or loaded pass in a location where the chain as it travels approaches the chain ring stack. The front derailleur likewise can be moved laterally so as to cause the chain to jump from one chain ring to another of different pitch diameter, thereby again changing the driving ratio.
Mechanical stops on both the front and rear derailleurs prevent the chain from being pushed off the extreme inboard and outboard sprocket rings of each stack.
A refinement of the derailleur system, generally called index shifting, utilizes detents in the lever mechanism to which the cable operating either the rear or the front derailleur is connected. The detents are spaced such that the interval through which the derailleur is moved laterally ideally transfers the chain accurately from one sprocket ring to another selected by the rider.
The system summarized above has the following four disadvantages in particular:
(1) A roller chain, while able to flex freely in the plane in which it and its entrained sprockets operate, is not designed to flex in a direction perpendicular to that plane. Hence, exerting a lateral force on the chain, i.e., a force perpendicular to the plane of operation, creates undesirable stresses and produces excessive wear in the chain.
(2) The lever and cable mechanisms which operate the front and rear derailleurs are subject to wear and dimensional distortion. Further, because of the mechanical advantage inherent in the parallelogram linkages which allow both rear and front derailleur to move laterally, a very small travel of the cable produces a relatively large movement of the derailleur. Consequently, the mechanisms are sensitive and prone to needing readjustment, particularly if subjected to frequent or heavy use.
(3) In transferring from one sprocket to another, the chain rollers usually do not fall immediately in the grooves of the newly entrained sprocket. More often than not, the rollers must ride over the crests of several successive sprocket teeth before falling in the grooves. It is not unusual for a receiving sprocket to rotate through more than 270.degree. before the chain meshes with it properly. Not only does this aggravate chain and sprocket wear, but it creates a lack of shifting precision and an undesirable distance of bicycle travel before the newly selected drive ratio is achieved.
(4) Index shifting works reasonably well on the free-wheel sprockets. This is not only because it operates on the slack pass of the chain, but also because the free-wheel sprockets are smaller in pitch diameter than the chain rings and therefore the transfer from one sprocket to an adjacent one takes place through a relatively small radial displacement. With the chain ring stack, however, accurate index shifting is more difficult. First of all, the transfer must be done with the tight pass of the chain, i.e., the pass which transmits the driving load. This makes lateral deflection of the chain more difficult. Moreover, the radial displacement from one chain ring to an adjacent one is much greater than it is with the free-wheel sprockets. This greater radial displacement makes chain transfer particularly difficult when transferring from a smaller to a larger diameter chain ring. Such a transfer normally requires that the chain be laterally deflected more to effect the transfer than the lateral displacement which is required once the transfer has been achieved. The front derailleur may therefore continue to drag against the chain after transfer, and achieving the transfer is often accompanied by an extended period during which the chain and sprocket are not properly meshed, and by undesirable chatter of the chain before it finally transfers to the receiving sprocket.
Alternatives to the derailleur system have been proposed, for example in Browning U.S. Pat. Nos. 4,127,038, 4,894,046 and 5,073,152, and in Browning et al U.S. Pat. No. 4,580,997. These Browning and Browning et al patents disclose bicycle shifting mechanisms which involve hinged sprocket sectors arranged such that the hinged sprocket sector can pick up the chain load from an adjacent sprocket in a sprocket stack, and have the characteristic of being able to shift under load. A disadvantage of the Browning and Browning et al mechanisms, however, is that a relatively wide space in the axial direction is required if a number of individual sprockets are to be included for a range of gear ratio. There is limited axial space available in a bicycle; a typical standard rear wheel fork is only 130 mm in width. Another disadvantage of some of the Browning and Browning et al mechanisms is a tendency to force a sharp lateral bend of the chain at the hinge point. Yet another disadvantage is that the relatively complex mechanism involved is not enclosed, making it more susceptible to contamination.